Capacitive microphone sensor design and fabrication method for achieving higher signal to noise ratio

ABSTRACT

A capacitive transducer or microphone includes a first substrate of one or more layers and which includes a first surface, a first cavity in the first surface, and a mesa diaphragm that spans the first cavity. The capacitive transducer or microphone includes a second substrate fixed to the first substrate. The second substrate has one or more layers which includes a second cavity having a nonplanar (e.g., contoured or structured or stepped) bottom surface that faces the mesa diaphragm. A shape or relief of the bottom surface of the cavity may advantageously be, to at least some degree, complementary to a deformed shape of the diaphragm. The second substrate may include one or more acoustic holes, non-uniformly distributed thereacross. One or more vents may vent the second cavity.

FIELD OF DISCLOSURE

The disclosure generally relates to designs for microphones. Morespecifically, the disclosure is related to capacitive microphones and inparticular designs and manufacturing processes that employ semiconductormanufacturing operations to achieve high signal to noise ratio.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

As the demand for sophistication is growing in communication devices,the same demand thus requires all components which are associated withmaking of these devices to be increasing in its standards. One of thesecommunication devices is the mobile phone. There are numerous componentsthat are included a single mobile phone. Among these components, someare fundamental to the device and without them the communication devicesare virtually useless. One of those components is the microphone. Themicrophone is an essential component in any communication device thatrequires vocal input. The performance of microphone along with few othercomponents dictates the mobile phone sound quality. Therefore, thedemand for higher performance microphones is a constant requirement bymobile phone manufacturer.

A microphone is an ultra-low-pressure sensor that senses incoming soundpressure that causes mechanical vibration of air molecules. The soundpressure is typically in the range of a few Pascals. There are a fewtransduction mechanisms that can be utilized to sense the soundpressure. Those mechanisms may include piezoresistive, piezoelectric,and capacitive sensing. Each of these mechanisms has its own advantagesand disadvantages in terms of microphone functionality. Capacitivesensing and piezoelectric sensing are more suitable for low pressure,high sensitivity sensing because their signal input to signal outputratio is higher than that of piezoresistive sensing.

There are few key performance parameters in microphones. These aresensitivity, frequency flatness and range, total harmonic distortion(THD), overload pressure, and signal to noise ratio (SNR). Among theseparameters, SNR is a key parameters as well as a dominant parameters indefining sound quality. The SNR determines the quality of the sound overthe whole operating range. Designing a microphone to get higher SNR isvery challenging. Thus, the incumbent microphone used in the mobilephone industry is limited with −65 dB SNR. Achieving higher SNR withtraditional designs is limited. Enhancing SNR performance of amicrophone while maintaining competitive cost requires new innovativeideas.

SUMMARY

Fabrication of piezoelectric mechanisms have a variety of shortcomings.Microphone sensitivity require, in general, a very thin diaphragm forsensing the micro vibration of air molecules caused by the soundpressure. Piezoelectric mechanisms typically employ a piezoelectricmaterial placed directly on the diaphragm for sensing the microvibration of air. The structural overlapping of a diaphragm andpiezoelectric material causes stress in the diaphragm, resulting in adecrease in sensitivity.

In the case of capacitive sensing, the traditional design has variousshortcomings in terms of performances. In general, high sensitivity isachieved with shorter distance between two conductive plates. Butshorter distance between two conductive plates increases leakingcurrent, which increases the white noise level and reduces the signal tonoise (SNR) ratio. The embodiments disclosed herein address variousshortcomings for capacitive sensing to achieve higher performance.

Aspect 1. A capacitive micro-electromechanical transducer, comprising: afirst substrate comprised of two or more layers and having an exteriorsurface, an interior surface, and a first cavity with a first opening atthe exterior surface of the first substrate and a second opening atleast proximate the interior surface of the first substrate, at leastone of the layers of the first substrate comprising a mesa diaphragmthat is electrically conductive and that extends across the secondopening of the first cavity, at least a portion of the mesa diaphragmmoveable along an oscillation axis; and a second substrate comprised ofat least one layer and having an exterior surface, an interior surface,and a second cavity with an opening at least proximate the interiorsurface of the first substrate and a cavity bottom surface, the interiorsurface of the second substrate secured to the interior surface of thefirst substrate with the mesa diaphragm in registration with the openingwith at least a portion of the mesa diaphragm positioned to oscillatealong the oscillation axis between the first and the second cavities,where at least a portion of the second substrate is electricallyconductive and the cavity bottom surface is non-planar.

Aspect 2. The capacitive micro-electromechanical transducer of Aspect 1wherein a depth of the second cavity as measured perpendicularly fromthe second opening to the cavity bottom surface increases as the cavityis laterally traversed from a perimeter thereof to a center thereof.

Aspect 3. The capacitive micro-electromechanical transducer of Aspects1-2 wherein the cavity bottom surface comprises a plurality of steppedregions, a depth of the second cavity as measured perpendicularly fromthe second opening to the cavity bottom surface increases as the cavityis laterally traversed from a perimeter thereof to a center thereof.

Aspect 4. The capacitive micro-electromechanical transducer of Aspects1-3 wherein, at least when static, the mesa diaphragm extendslongitudinally inwards along the oscillation axis, toward the cavitybottom surface.

Aspect 5. The capacitive micro-electromechanical transducer of Aspects1-4 wherein a maximum perpendicular distance between mesa diaphragm andthe cavity bottom surface renders a value of capacitance of thecapacitive micro-electromechanical transducer independent of a thicknessof an electrical isolation.

Aspect 6. The capacitive micro-electromechanical transducer of Aspects1-5 wherein the second substrate includes a plurality of holes thatextend from the cavity bottom surface through the exterior surface ofthe second substrate.

Aspect 7. The microphone of Aspects 1-6 wherein the plurality of holesthat extend from the cavity bottom surface through the exterior surfaceof the second substrate are non-uniform in at least one of size ordistribution, where a relative density of the holes or a relative sizeof the holes increasing as the cavity bottom surface is laterally orradially traversed from a perimeter thereof to a center thereof.

Aspect 8. The microphone of Aspects 1-7 wherein the holes of theplurality of holes are uniform is size in a lateral dimension, and adensity of the holes is higher at a center than at a perimeter of thecavity bottom surface.

Aspect 9. The capacitive micro-electromechanical transducer of Aspects1-8 wherein at least one of the first or the second substrates includesa vent that extends laterally from the second cavity to an exterior ofat least one of the first or the second substrates to vent a chamberformed by the cavity and the mesa diaphragm.

Aspect 10. The capacitive micro-electromechanical transducer of Aspects1-9 wherein the first substrate includes an electrically conductive lineelectrically coupled to the mesa diaphragm.

Aspect 11. The capacitive micro-electromechanical transducer of Aspects1-10 wherein the first substrate includes an electrically conductiveline electrically coupled to the electrically conductive portion of thesecond substrate.

Aspect 12. The capacitive micro-electromechanical transducer of Aspects1-11 wherein the first substrate includes a wafer and at least one oxidelayer carried by the wafer at least proximate the inner surface of thefirst substrate.

Aspect 13. The microphone of Aspects 1-12 wherein the first substrateincludes a wafer and a first oxide layer carried by the wafer, and thesecond substrate includes an electrically conductive or semiconductivelayer and a second oxide layer carried by the electrically conductive orsemiconductive layer.

Aspect 14. The capacitive micro-electromechanical transducer of Aspects1-13 further comprising: a fusion bond that secures the second substrateto the first substrate via the first and the second oxide layers.

Aspect 15. A microphone, comprising: a capacitivemicro-electromechanical transducer; and a packaging that houses thecapacitive micro-electromechanical transducer, the housing having atleast two contacts on an exterior thereof, wherein the capacitivemicro-electromechanical transducer comprises: a first substratecomprised of two or more layers and having an exterior surface, aninterior surface, and a first cavity with a first opening at theexterior surface of the first substrate, at least one of the layers ofthe first substrate comprising a mesa diaphragm that is electricallyconductive and that spans the first cavity, at least a portion of themesa diaphragm moveable along an oscillation axis with respect to atleast one layer of the first substrate; and a second substrate comprisedof at least one layer and having an exterior surface, an interiorsurface, and a second cavity with a second opening at least proximatethe interior surface of the second substrate and a cavity bottomsurface, the interior surface of the second substrate secured to theinterior surface of the first substrate with the mesa diaphragm inregistration with the second opening with at least a portion of the mesadiaphragm positioned to oscillate along the oscillation axis at leastpartially into the second cavity, where at least a portion of the secondsubstrate is electrically conductive and the cavity bottom surface isnon-planar.

Aspect 16. The microphone of Aspect 15, further comprising: acapacitance sensor circuit electrically coupled to the mesa diaphragmand electrically coupled to at least the portion of the second substratethat is electrically conductive to sense a change in capacitance as themesa diaphragm vibrates.

Aspect 17. A method to fabricate a capacitive micro-electromechanicaltransducer, the method comprising: forming a mesa diaphragm in a layerof a first substrate, the mesa diaphragm spanning a first cavity in atleast a second layer of the first substrate, the first cavity open at anexterior surface of the first substrate, the exterior surface opposed toan interior surface of the first substrate; providing a second substratehaving an interior surface, an exterior surface, and a second cavityformed in the interior surface of the second substrate, the secondcavity having a second opening and terminating in a cavity bottomsurface, the cavity bottom surface being non-planar across at least onelateral or radial dimension thereof attaching the interior surface ofsecond substrate to the interior surface of the first substrate with themesa diaphragm in registration with the second cavity and with the mesadiaphragm oscillatable along a longitudinal axis at least partially intothe second cavity.

Aspect 18. The method of Aspect 17, further comprising: patterning thecavity bottom surface.

Aspect 19. The method of Aspects 17-18 wherein patterning the cavitybottom surface includes forming a plurality of stepped regions, a depthof the second cavity as measured from the second opening to the cavitybottom surface increases as the cavity bottom surface is laterally orradially traversed from a perimeter thereof to a center thereof.

Aspect 20. The method of Aspects 17-19, further comprising:

forming a plurality of holes in the second substrate that extend fromthe cavity bottom surface through the exterior surface of the secondsubstrate.

Aspect 21. The method of Aspects 17-20 wherein forming a plurality ofholes in the second substrate includes forming the plurality of holeswhich are non-uniform in at least one of size or distribution, where arelative density of the holes or a relative size of the holes increasingas the cavity bottom surface is laterally or radially traversed from aperimeter thereof to a center thereof.

Aspect 22. The method of Aspects 17-21 wherein forming a plurality ofholes in the second substrate includes forming the plurality of holeswhich are uniform in size a lateral dimension, and a density of theholes is higher at a center than at a perimeter of the cavity bottomsurface.

Aspect 23. The method of Aspects 17-22, further comprising: forming atleast one vent in at least one of the first or the second substratesthat extends laterally from the second cavity to an exterior of at leastone of the first or the second substrates to vent a chamber formed bythe second cavity and the mesa diaphragm when the first and the secondsubstrates are attached together.

Aspect 24. The method of Aspects 17-23 wherein attaching the interiorsurface of second substrate to the interior surface of the firstsubstrate includes fusion bonding the interior surface of secondsubstrate to the interior surface of the first substrate.

In at least one embodiment, the diaphragm includes polysilicon materialand non-polysilicon materials. In another embodiment, the fusion bondingsurface of the diaphragm does not contain any polysilicon film. Inanother embodiment, the fusion bonding surface at either plate, does notcontain polysilicon film. In at least one embodiment, the fusion bondingtemperature must be above 1000° C. in Oxygen (O₂) environment. Inanother embodiment, a thin layer of SiO2 can be deposited at the sectionof polysilicon prior to fusion bonding and can bond at lower temperatureand create mesa structure. In at least one embodiment, the diaphragm canbe in mesa formation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not necessarily drawn to scale, and some ofthese elements are arbitrarily enlarged and positioned to improvedrawing legibility. Further, the particular shapes of the elements asdrawn are not necessarily intended to convey any information regardingthe actual shape of the particular elements, and have been solelyselected for ease of recognition in the drawings. Where a referenceframe is provided (e.g., top/bottom) on a drawing sheet for a givenFigure, that reference frame applies only to that Figure, and notnecessarily to other Figures.

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures, andwherein:

FIG. 1 is a schematic view of a capacitor and power source according toone embodiment of the disclosure.

FIG. 2 is a schematic view of a capacitor with an isolation layer and apower source according to one embodiment of the disclosure.

FIG. 3 is a schematic view of a capacitor and a power sourceillustrating a polarization a pair of plates of the capacitor accordingto one embodiment of the disclosure.

FIG. 4 is a schematic view of a capacitor with at least one dielectricmaterial filling at least a portion of a gap between plates of thecapacitor according to one embodiment of the disclosure.

FIG. 5 is a schematic view of a capacitor according to an embodiment ofthe disclosure.

FIG. 6 is a schematic view of a capacitive microphone sensor accordingto one embodiment of the disclosure.

FIG. 7 shows a capacitive microphone according to one embodiment of thedisclosure.

FIGS. 8A-8E illustrate a method to manufacturing a capacitive microphonewith a mesa diaphragm using various fabrication operations according toone embodiment of the disclosure.

FIG. 9A shows a sectional view of a mesa diaphragm according to oneembodiment of the disclosure.

FIG. 9B shows a sectional view of a microphone structure that has a mesadiaphragm according to one embodiment of the disclosure.

FIG. 10 shows a sectional view of a microphone structure that has a mesadiaphragm according to one embodiment of the disclosure.

FIG. 11 shows a sectional view of a microphone with a box cavity designaccording to one embodiment of the disclosure.

FIG. 12 shows a sectional view of a capacitor with a contoured cavitydesign according to one embodiment of the disclosure.

FIG. 13 shows a sectional view of a capacitor with a contoured cavitydesign according to one embodiment of the disclosure.

FIG. 14 shows a sectional view of a capacitor with a contoured cavitydesign and a mesa diaphragm according to one embodiment of thedisclosure.

FIGS. 15A-15O show a method of make a capacitor with a contoured cavitydesign and a mesa diaphragm according to one embodiment of thedisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, some specific details are included toprovide a thorough understanding of various disclosed embodiments. Oneskilled in the relevant art, however, will recognize that embodimentsmay be practiced without one or more of these specific details, or withother methods, components, materials, fabrication operations, etc. Inother instances, well-known structures and fabrication operationsassociated with integrated circuit fabrication have not been shown ordescribed in detail to avoid unnecessarily obscuring descriptions of theembodiments of the present methods. Throughout this specification andthe appended claims, the words “element” and “elements” are used toencompass, but are not limited to, all such structures, systems, anddevices associated with integrated circuit fabrication.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprising” is synonymous with“including,” and is inclusive or open-ended (i.e., does not excludeadditional, unrecited elements or method acts), as are variationsthereof, such as, “comprises” and “comprise.”

Reference throughout this specification to “one embodiment” “anembodiment”, “another embodiment”, “one example”, “an example”, “anotherexample”, “one implementation”, “another implementation”, or the likemeans that a particular referent feature, structure, or characteristicdescribed in connection with the embodiment, example, or implementationis included in at least one embodiment, example, or implementation.Thus, the appearances of the phrases “in one embodiment”, “in anembodiment”, “another embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment, example, or implementation. Furthermore, the particularfeatures, structures, or characteristics may be combined in any suitablemanner in one or more embodiments, examples, or implementations.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to a layerincludes a single layer, or two or more layers. It should also be notedthat the term “or” is generally employed in its non-exclusive sense,i.e., “and/or” unless the content clearly dictates otherwise.

The headings provided herein are for convenience only and do notinterpret the scope or meaning of the embodiments.

Working Principles of Capacitive Sensor

FIG. 1 shows a capacitor 100 a according to one embodiment of thedisclosure. FIG. 1 includes a first plate 102 and a second plate 104.The first plate 102 is coupled to a positive terminal of a power source106. The second plate is coupled to a negative terminal of the powersource 106. A cavity 108 exists between the first plate 102 and thesecond plate 104.

FIG. 2 shows a capacitor 100 b according to one embodiment of thedisclosure.

Similar to FIG. 1 , FIG. 2 includes a first plate 102 and a second plate104. The first plate 102 is coupled to a positive terminal of a powersource 106. The second plate is coupled to a negative terminal of apower source 106. A cavity 108 exists between the first plate 102 andthe second plate 104, wherein the cavity 108 is created by disposingelectrically isolation layer 110, e.g., silicon dioxide (SiO₂) betweentwo plates 102, 104. Ideally, the two plates 102 and 104 should beelectrically isolated and should have no current leakage (illustrated byarrow 112) between the two plates 102, 104. In reality, there is mostlikely a leakage current 112 between the two plates 102, 104. Thesmaller the current leakage 112, the better for the sensitivity of thecapacitor. The larger the current leakage 112, the more the sensitivityof the capacitor is degraded.

FIG. 3 shows the capacitor 100 b of FIG. 2 with a polarization of theplates 102, 104 illustrated. The first plate 102 may, for example have arelatively positive voltage while the second plate 104 may have arelatively negative voltage.

FIG. 4 shows a capacitor 110 c where the cavity 108 is at leastpartially filled with dielectric material according to one embodiment ofthe disclosure. The dielectric material 114 may take the form of a fluid(i.e., gas, liquid) or a solid. Different dielectric materials mayprovide different permittivity ε, which can be expressed as a constant.

FIG. 5 shows a capacitor 100 d according to an embodiment of thedisclosure. The capacitor 100 d includes a first plate 102 and a secondplate 104. The first plate 102 is coupled to a positive terminal of apower source 106. The second plate is coupled to a negative terminal ofa power source 106. A cavity 108 exists between the first plate 102 andthe second plate 104, wherein the cavity 108 is defined by the firstplate 102, second plate 104, and electrically isolation layer 110, e.g.,silicon dioxide (SiO₂).

The first plate 102 and the second plate 104 have a surface area A 202.The electrically isolation layer 110 created a distance d 204 betweenthe two plates 102, 104. The dielectric material 114 can, for example,take the form of air 206. Air 206 has a permittivity ε. The relationshipbetween capacitance C, surface area A 202, distance d 204, andpermittivity ε is C=εA/d. Thus, to have a larger capacitance, thesurface area A 202 should be larger, and/or the distance d 204 should besmaller, according to the equation of C=εA/d.

Reducing the distance d 204 leads to a higher capacitance C. Thus,theoretically, a thinner isolation layer 110 leads to betterperformances. However, a thinner isolation layer 110 leads to largercurrent leakage 112 which compromises the performance. On the otherhand, a thicker isolation layer 110 reduces the current leakage 112between the plates 102, 104. See also FIG. 2 . Therefore, one shouldfind a distance d 204 and thickness of the isolation layer 110 thatoptimizes the overall capacitor performance.

Capacitor Mechanism Transferred to Capacitance Sensing

Various embodiments described herein advantageously employ capacitancesensing transducers or sensors to sense the micro vibration of air(e.g., changes in pressure or sound) and to produce an electrical signalthat represents the sensed micro vibration of air.

FIG. 6 shows a capacitor 100 e with a first plate 102 that is a flexibleor moveable with respect to a second plate 104 of the capacitor.Notable, when the distance 204 between plates 102, 104 changes, thevalue of capacitance C changes. The distance 204 is inversionalproportional to capacitance value, according to C=εA/d. Therefore, forcapacitive sensing, at least one of the plates 102, 104 in the capacitor100 e is movable with respect to the other (in FIG. 6 , the movableplate is 102 which is also denominated as the diaphragm 102).Furthermore, a cavity 308 between two plates 102, 104 is at leastpartially filled with a gas, for example air. The cavity 308 is defined,at least partially, by the first plate 102, the second plate 104, andthe insulation layer 110. Therefore, diaphragm 102 has the space todeflect or move when required. The diaphragm 110 responds to incidentvibration of air by deforming in shape. This alteration of the distanced 204 between the plates 102, 104 changes the value of capacitance C(see FIG. 6 ). The change of capacitance C can be read out with variousmethods, e.g., the charging time of the capacitor, the voltage (V)change, or current (I) change. From the change in capacitance we canascertain the amplitude of the air pressure or sound.

Capacitance Sensing Utilized for Sound Sensing

The sensing of sound (air micro vibration) employs dynamic pressuresensing unlike typical static pressure sensing. Therefore, specificdesign features are required from the basic capacitive structure, atleast some of which design features are s shown in FIG. 7 .

A microphone 700 includes a diaphragm 702. The diaphragm 702 may takethe form of a first electrically conductive plate 102 that is flexibleor moveable. The microphone 700 also includes a second electricallyconductive plate 704. In some embodiments, the second electricallyconductive plate 704 may be a fixed plate, while the diagram 702 isflexible or moveable with respect to the second electrically conductiveplate 704. One or more isolation layers 706 are disposed between thediaphragm 702 and the second electrically conductive plate 704. A firstcavity 708 is defined, at least partially, by the diaphragm 702, thesecond electrically conductive plate 704, and the isolation layer 706.The isolation layer 706 includes at least one vent 710. The vent 710 mayextend in a direction parallel to a top and/or bottom surface of thediaphragm 702 and/or second electrically conductive plate 704. The vent710 fluidically couples the first cavity 708 with an exterior of themicrophone 700.

The second electrically conductive plate 704 includes a plurality ofacoustic holes 712. The acoustic holes 712 provide a number of passagesbetween the first cavity 708 and a second cavity 716.

A variable distance between the two plates 102, 104 is d 722. Theisolation layer 706 has a thickness h 724. When the diaphragm 702 isstatic, the thickness h 724 of the isolation layer 706 is the same asthe distance d 722. When the diaphragm 702 is vibrating, theperpendicular distance d 722 between a particular point of the diaphragm702 and the conductive plate 704 may vary. Each point of the diaphragm702 may have a different vibration amplitude. The vibration amplitudecan be the largest at a central area of the diaphragm 702 and laterallyreduced toward peripheral areas thereof. During vibration, a maximumperpendicular axial distance between a diaphragm 702 and the secondelectrically conductive plate 704 surface is within a defined range ofbetween 1 nanometer and 50 micrometer.

Embodiments of this disclosure offer unique microphone designs andfabrication sequences that can achieve improved SNR, for example an SNRexceeding −65 dB. There are many ways to construct or fabricatesemiconductor microphones. The capacitive sensor can be built in eithera single substrate or using multiple substrates. However, each approachhas its advantages. Furthermore, there are limitations in variousdesigns and fabrications.

A microphone includes a transducer or sensor, packaging or housing, andoptionally an amplifier circuit. Noise can originate from all threecomponents in which acoustic noise from the sensor and floor noise fromthe amplifier are typically the largest and most significant sources ofnoise.

Embodiments of this disclosure are directed to the transducer, and inparticular to reducing noise associated with the transducer which issometimes referred to as acoustic noise. Ideally, acoustic noise shouldbe reduced to zero if possible. The acoustic noise depends on a fewparameters, including (a) leakage current between the plates; (b) gapsize between the plates; and (c) acoustic hole density. These parametersare interdependent parameters and adjusting one parameter to improveperformance may adversely affect another one of the parameters reducingperformance.

As shown in FIG. 7 , increasing the thickness h 724 of the isolationlayer 706, will reduce acoustic noise because current leakage isreduced. However, increasing the thickness h 724 will reduce thecapacitance C, where capacitance is given by the formula C=εA/d, andwhen distance d is increased the capacitance C is reduced. As describedbelow, some structures (e.g., contour cavity or step cavity) can be usedto balance or at least partially offset the adverse effect oncapacitance C that results from increasing the thickness h 724.

Increasing a total number and/or a total volume or total area ofacoustic holes 712 can reduce acoustic noise. Such an increase will,however, reduce plate area A and thus adversely reduce capacitance C,where C is given by the formula C=εA/d. Embodiments that include thedistributed acoustic holes 712 (highest density toward the central areaand laterally reduced densities toward peripheral areas) describedherein can at least partially offset the adverse effect on capacitance Cthat results from increasing number, size and/or area of acoustic holes712, which would otherwise reduce sensitivity. In at least someembodiments, acoustic holes 712 can be distributed with varyingdensities. For example, the density of the acoustic holes mayadvantageously be highest at or toward a center of the cavity andlaterally reduced at or towards peripheral areas. Additionally oralternative, the acoustic holes can be distributed to advantageouslyvary the total open or ported area across a lateral dimension. Forexample, a total open or ported area may be relatively larger at ortoward a center of the cavity and laterally reduced smaller at ortowards peripheral areas.

As previously noted, increasing the isolation layer thickness h 724,reduces leakage current between plates, which in turn reduces acousticnoise. However, on the other hand, increasing the isolation layerthickness h 724 will increase the distance d 722 between both plates702, 704 thereby reducing the capacitance C, thus adversely impactingthe sensitivity. Various embodiments of this disclosure can overcomethis counter effect. In various embodiments with a mesa diaphragm designas taught herein, the specific structural designs can advantageouslymake the capacitance C value independent of the thickness of theisolation layer h 724.

In at least one embodiment, the two electrically conductive plates 702,704 are made from respective substrates. The substrates comprisedifferent types of material from one another.

For example, a diaphragm 702 may be made, at least partially, frompolysilicon substrate, while the second electrically conductive plate704 may be made from non-polysilicon substrate.

In at least one embodiment, the second electrically conductive plate704, which may be fixed, is formed as a stepped cavity structure. In atleast one embodiment, the diaphragm 702 is formed inside the steppedcavity.

In at least one embodiment, the diaphragm 702 and the secondelectrically conductive plate 704 are bonded together, for example viathe isolation layer by fusion bonding.

In at least one embodiment, the diaphragm 702 includes both polysiliconmaterial and non-polysilicon material. In at least one embodiment, afusion bonding surface of the diaphragm 702 does not contain anypolysilicon film. In at least one embodiment, the fusion bonding surfaceat either diaphragm 702 and/or second electrically conductive plate 704does not contain a polysilicon film. In at least one embodiment, thefusion bonding temperature must be above 1000° C. in an Oxygen (O₂)environment. In at least one embodiment, a thin layer of SiO2 can bedeposited at the section of polysilicon prior to fusion bonding, and canthus advantageously bond at lower temperature and create mesa structure.

Mesa Diaphragm Design and Manufacture

FIGS. 8A-8E shows a method 800 of producing a microphone with adiaphragm mesa formation according to at least one embodiment of thedisclosure.

As illustrated in FIG. 8A, the method 800 includes, at least, preparinga first semiconductor substrate 820. Method 800 may include etching atleast a portion of a top surface of the first semiconductor substrate820 to form a cavity 832. The cavity 832 can have a depth of, forexample, 2 μm-10 μm recessed into the semiconductor substrate 820. In atleast one embodiment, the cavity 832 is recessed 4 μm deep into thefirst semiconductor substrate 820.

Method 800 includes depositing a first silicon dioxide layer 810 on thetop surface of the first semiconductor substrate 820. The first silicondioxide 810 layer can have a thickness, for example, of 0.5 μm-6 μm. Inat least one embodiment, the first silicon dioxide layer has a thicknessof 2 μm. The first silicon dioxide layer 810 covers a top surface withinthe recessed cavity as shown in FIG. 8A.

The method 800 includes depositing a first polysilicon layer 802 on thetop surface of the first semiconductor substrate 820 within the recessedcavity. This polysilicon layer 802 constitutes the diaphragm thatvibrates due to sound waves. The polysilicon layer 802 is deposited on atop surface of the silicon dioxide 810 layer in the cavity 832 as shownin FIG. 8A.

As illustrated in FIG. 8B, the method 800 includes preparing a secondsemiconductor substrate 804. In at least one embodiment, the methodincludes etching a second cavity 831 into the bottom surface of thesecond semiconductor substrate 804, such that when the secondsemiconductor substrate is bonded with the first semiconductorsubstrate, the second cavity 831 and the first cavity 832 arefluidically connected. The dash line in FIG. 8B and FIG. 8C shows theseparation (e.g., an imaginary boundary) of the first cavity 832 and thesecond cavity 831. As shown in FIG. 8C, the first cavity 832 is recessedinto the first semiconductor substrate 820. The second cavity 831 isrecessed into the second semiconductor substrate 804.

As further illustrated in FIG. 8B, the method 800 includes depositing asecond silicon dioxide layer 812 on a bottom surface of the secondsilicon semiconductor substrate 804. In at least one embodiment, thesecond silicon dioxide layer 812 is not deposited in the cavity 832. Thesecond silicon dioxide layer 812 can have a thickness, for example, of 1μm-5 μm. In at least one embodiment, the second silicon dioxide layer812 has a thickness of 2.5 μm.

As further illustrated in FIG. 8B, the method 800 includes fusionbonding of the bottom surface of the second silicon dioxide layer 812and a top surface of the first silicon dioxide layer 810, forming abonding surface 814. Fusion bonding is also known as thermal bonding.Fusion bonding includes increasing the temperature to a point that thesilicon dioxide molecules at or around the bottom surface of the secondsilicon dioxide layer 812 and a top surface of the first silicon dioxidelayer 810 (e.g., bonding surface) fuse into each other and formscovalent bonds between the first 810 and the second 812 silicon dioxidelayers. In at least one embodiment, the temperature for fusion bondingis, for example, about 950° C. to 1150° C. In at least one embodiment,the temperature for fusion bonding is, for example, 1050° C.

As illustrated in FIG. 8C, the method 800 includes grinding or polishing(e.g., chemical-mechanical planarization) the second semiconductorsubstrate 804 to a thickness, for example, of 10 μm-30 μm. In at leastone embodiment, the second semiconductor substrate 804 has a thicknessof 15 μm after grinding or polishing.

As shown in FIG. 8D, the method 800 includes etching bottom cavity 834from a bottom surface of the first semiconductor substrate 820. Thisbottom cavity 834 will merge into and end up being part of the firstcavity 832. At FIG. 8D, the bottom cavity 834 is defined, at leastpartially, by the first semiconductor substrate 820 and the firstsilicon dioxide layer 810. The bottom cavity 834 is fluidically coupledto an exterior of the microphone. In at least one embodiment, the bottomcavity 834 includes an opening to the exterior of the microwave toreceive the sound pressure waves. The dash line in FIG. 8D shows theseparation (e.g., an imaginary boundary) of the first cavity 832 and thesecond cavity 831.

As illustrated in FIG. 8E, the method 800 includes etching away thefirst silicon dioxide layer 810 from a bottom side of the first silicondioxide layer 810. The first silicon dioxide layer 810 in the bottomcavity 834 is being etched away. In FIG. 8E, the bottom cavity 834 ismerged into and becomes part of the first cavity 832. The first cavity832 is now fluidically coupled to an exterior to the capacitive acoustictransducer package or microphone. In at least one embodiment, the firstcavity 832 includes an opening to the exterior of the microphone toreceive the sound pressure waves. The dash line in FIG. 8E shows theseparation (e.g., an imaginary boundary) of the first cavity 832 and thesecond cavity 831.

After the first silicon dioxide layer 810 is etched away, thepolysilicon layer that comprises the diaphragm 802 is released. Inresponse to releasing the diaphragm 802, the diaphragm 802 flips up andforms a mesa shape, i.e., a mesa diaphragm. The diaphragm 802 extendstowards and into the second cavity 831, as shown in FIG. 8E. Thediaphragm 802 extends inwardly into the second cavity 831.

The diaphragm 802 vibrates 850 in response to receipt of sound pressurewaves. This means the value of capacitance C changes or variesdynamically (i.e., over time). The changes of the value of capacitance Ccan be detected electronically. The changes of the value of capacitanceC represents one or more characteristics (e.g., amplitude, frequency) ofthe sound pressure waves received. The mesa diaphragm 802 is structuredsuch that the diaphragm 802 does not touch a bottom surface of thesecond semiconductor substrate 804 or when the diaphragm 802 vibrates.

FIG. 9A is a sectional view of a mesa diaphragm 902 according to oneembodiment of the disclosure. A cross-sectional shape of the mesadiaphragm 902 is similar in shape to a trapezoid without a base. Themesa diaphragm 902 has a flat top boarder 970, and two sloping sideboarders 972 in a sectional view.

The mesa diaphragm 902 includes an inner surface 990. In at least oneembodiment, the inner surface 990 faces the first cavity 934 (FIG. 9B).The mesa diaphragm 902 includes an outer surface 992. In at least oneembodiment, the outer surface 992 faces the second cavity 932 (FIG. 9B).

As best illustrated in FIG. 9A, the mesa diaphragm 902 when un-deformedhas a profile (cross-section) that is similar to a trapezoid without abase. The mesa diaphragm 902 includes a top boarder 970 and two sideboarders 972. The bottom ends of the side boarders 972 have anchors 956.In at least one embodiment, the anchors 956 attach to the first silicondioxide layer 910 (FIG. 9B). The side boarder 972 joins the top boarder970 at a corner 962. The corner 962 has an angle α 974 measured from theinner surface 990. In at least one embodiment, the angle α 974 is largerthan 70°, 80°, 90°, 100°, or 110°.

FIG. 9B shows a design of a microphone with mesa diaphragm according toat least one embodiment of the disclosure.

As shown in FIG. 9B, the microphone 901 includes a mesa diaphragm 902.The mesa diaphragm vibrates 950 when sound pressure waves 960 reach themesa diaphragm 902. The mesa diaphragm 902 has one or more anchors 956attached to the first silicon dioxide layer 910. The center portion ofthe mesa diaphragm 902 can freely vibrate with as the sound pressurewave 960 varies in frequency and amplitude. The center portion of themesa diaphragm 902 vibrates in a direction that is perpendicular to atop or bottom surface of the mesa diaphragm 902. The diaphragm 902 isstructured and positioned such that the diaphragm 902 does not contactthe second semiconductor substrate 904 during vibration 950, maintainingthe characters of a capacitor.

In at least one embodiment, because the mesa diaphragm 902 is veryflexible and anchored to the first silicon dioxide layer 910, when mesadiaphragm 902 is vibrating 950, at least a portion of the top boarder970 of the mesa diaphragm 902 may form arcuate surfaces (e.g., concaveand/or convex), similar to diaphragm 102 in FIG. 6 .

In at least some embodiments, the corners 962 are formed or exist whenthe diaphragm is static. In at least one embodiment, during themanufacturing process, e.g., method 800, the corners 962 are formed whenthe first isolation layer 910 (see also e.g., 810) is etched away from abottom surface of the diaphragm 902. When the first isolation layer 910(see also e.g., 810) is etched away from a bottom surface of thediaphragm 902, the diaphragm 902 flips upward and extends toward thesecond substrate 904 forming the mesa structure as shown in FIGS. 9A and9B. Thus, in such embodiments, the corners 962 exist when the mesadiaphragm 902 is static (not deformed).

The first silicon dioxide layer 910 and the second silicon dioxide layer912 are bonded 954 together (the arrow of 954 shows the bondingorientation). The top surface of the first silicon dioxide layer 910 isa bonding surface. The bottom surface of the second silicon dioxidelayer 912 is another bonding surface. The two bonding surfaces arecovalently bonded through fusion bonding process, e.g., baking thedevice for example at a temperature of about 950° C. to 1200° C. for aperiod of time. In at least one embodiment, the baking temperature is1050° C. Under such temperature(s), the silicon dioxide molecules inboth silicon dioxide layers fuses across the bonding surface. Therefore,the top surface of the first silicon dioxide layer 910 and the bottomsurface of the second dioxide layer 912 are covalently bonded.

The second silicon dioxide layer 912 includes a vent hole 964 thatextends through the second silicon dioxide layer 912. The vent hole 964fluidically couples the second cavity 932 and an exterior of themicrophone 901. The vent hole 964 runs, for example, in a directionparallel to a top and/or bottom surface of the first 920 and/or second904 semiconductor substrate.

The second semiconductor substrate 904 includes one or more vent holes963 The vent hole 963 fluidically couples the second cavity 932 and anexterior of the microphone 901. The vent holes 963 run, for example, ina direction perpendicular to a top and/or bottom surface of the first920 and/or second 904 semiconductor substrate.

The first cavity 934 is recessed into the first semiconductor substrate904. The first cavity 934 is fluidically coupled to an exterior of themicrophone.

FIG. 10 shows a sectional view of a microphone according to at least oneembodiment of the disclosure.

As shown in FIG. 10 , there is a distance G between a top surface of themesa diaphragm 902 and a bottom surface of the second semiconductorsubstrate 904; a distance d₁ between a bottom end (anchors) of the mesadiaphragm 902 and a bottom surface of the second semiconductor substrate904 in the second cavity 932; a distance d₂ between a bottom end(anchors) of the mesa diaphragm 902 and the bottom surface of the topboarder of the mesa diaphragm 902; and a thickness t of the mesadiaphragm 902. The distance G and the distance d₂ varies as the mesadiaphragm 902 vibrates over time and varies across one or moredimensions (e.g., radial dimensions) of the mesa diaphragm 902 at anygiven time during vibration. The relationship between G, d₁, d₂, and tcan be expressed as: G=d₁−(d₂+t).

S₁ is the thickness of the first silicon dioxide layer 910; do is thedistance between a bottom end of the first silicon dioxide layer 910 anda bottom surface of the second semiconductor substrate 904 in the secondcavity 932. The relationship between d₁, do, and S₁ can be expressed as:d₁=d₀−S₁.

C₁ is the depth of a recession. The recession is a portion of the firstcavity 934 made by an etching operation from a top surface of the firstsemiconductor substrate 920. See 832 at FIG. 8A for example. C₁ is thedistance between a bottom surface of the recession and a top surface ofthe first semiconductor substrate 920. S₂ is the thickness of the secondsilicon dioxide layer 912. The relationship between d₀, C₁, S₁, and S₂can be expressed as: d₀=C₁+S₁+S₂.

Since d₁=d₀−−S₁ and d₀=C₁+S₁+S₂, thus d₁=(C₁+S₁+S₂)−S₁. Therefore,d₁=C₁+S₂.

Since G=d₁−(d₂+t) and d₁=C₁+S₂, thus G can be expressed asG=(C ₁ +S ₂)−(d ₂ +t)  Eq. 1

The Eq. 1 shows the thickness of the first isolation layer (S₁) 910 isindependent of the value of G, where G is the distance between a topsurface of the mesa diaphragm 902 and a bottom surface of the secondsemiconductor substrate 904. This means the first isolation layer (S₁)910 can be any thickness without having an effect of G. This also meanswith the mesa diaphragm design shown in FIG. 10A, the S₁ can beincreased for the purpose to reduce leaking current without compromisingon the sensitivity of the microphone.

In addition, the thickness S₂ of the first silicon dioxide layer 910 canbe maximized by adjusting the values of C₁ and d₂. According to Eq. 1:G=(C₁+S₂)−(d₂+t), the effect on G due to an increase of S₂ for a certainamount can be cancelled by a reduction of C₁ for the same amount, or inalternative an increase of d₂ for the same amount. Increasing S₂ canreduce the leaking current noise which in turn increasing thesensitivity of the microphone. Thus, the mesa diaphragm design shown inFIGS. 8A, 8B, 9A, 9B, and 10 provides the structural design that allowsthe microphone device to have very low leaking current noise, with no orinsignificant impact on the microphone's sensitivity.

Contour Cavity Design

As previously shown, the capacitance plates (movable and fixed plates)are made in two separate conductive and/or semiconductive substrates.The movable plate (diaphragm) is formed in one of the substrates. Thefix plate is formed in a separate substrate. These two separates platesare combined with a dielectric (e.g., air) there between to constitute acapacitor. The two separate substrates are attached to each other, forexample through fusion bonding.

Combining a contour cavity design with mesa diaphragm improvesmicrophone performance with reduced acoustic noise. When sound pressurewaves impinge on diaphragm 1104, the diaphragm 1104 bends or deforms inthe form shown in FIG. 11 .

As shown in FIG. 11 , the peripheral area 1102 of the diaphragm 1104hardly moves. About ⅓ of total area of diaphragm 1104 that is proximatethe peripheral area 1102 has very little or no movement. Most, if notall, of the vibrations of the diaphragm 1104 occur in the central area1103. The peripheral area 1102 does not significantly contribute tooverall sound pressure sensing except for the basic capacitorconstruction. Therefore, embodiments disclosed herein modify the flatparallel plate concept to a contour parallel plate concept in which aportion (e.g., bottom surface of cavity) of the fix plate 1106 ismodified to be complementary to the deflection of diaphragm 1104 tomaximize the capacitance changes when the diaphragm 1104 vibrates.

FIG. 12 shows a sectional view of a contour cavity design according toat least one embodiment of the disclosure. In FIG. 12 , the contour ofthe fix plate 1206 is implemented as a stepped contour surface 1208. Astepped contour surface 1208 can be manufactured in a semiconductorprocessing. It is noted that FIG. 12 does not present a limitation toany of the embodiments in this disclosure to include a smooth concavesurface at a top surface of the fixed plate facing the diaphragm. Forexample, the step size can be reduced within the limits of theparticular fabrication process being used, and the total number of stepsincreased to at least approach a relatively smooth contour or curvaturewhere distinct steps in the surface become undetectable via unaidedhuman vision.

The stepped contour surface 1208 has at least two levels of steps,wherein the bottom level 1210 (the lowest level) of the stepped contoursurface 1208 is centered around or facing a central area 1220 of thediaphragm 1204 that has the largest vibration magnitude or amplitude.Here, the largest vibration magnitude or amplitude of the central area1220 is in comparison to the peripheral area 1221 of the diaphragm 1204.The area 1220 includes a particular point of the diaphragm 1204, e.g.,the center point, that provides the largest vibration magnitude oramplitude when the diaphragm 1204 receives the sound wave pressure. Inone at least one embodiment, the area 1220 of the diaphragm 1204 thathas the largest vibration magnitude or amplitude is at or around acentral area of the diaphragm 1204.

As shown in FIG. 12 , the stepped contour surface 1208 cavity includesthree steps 1210, 1212, 1214. The lowest level step is 1210 is facing acentral area of the diaphragm 1220. The highest level step 1214 isfacing the peripheral area 1221 of the diaphragm 1214. In someembodiments, the highest level step 1214 is faces against the anchors ofthe diaphragm 1204. The middle level step 1212 is disposed between thelowest level step 1210 and the higher level step 1214.

FIG. 13 shows a sectional view of a contour cavity design with ventholes according to one embodiment of the disclosure. In FIG. 13 , thecontour of the fix plate 1304 is a stepped contour surface 1308.

The stepped contour surface 1308 has three levels of steps 1310, 1312,1314, wherein the bottom level 1310 (the lowest level) of the steppedcontour surface 1308 is centered around or facing an area 1320 of thediaphragm 1302 that has the largest vibration magnitude or amplitude.Here, the largest vibration magnitude or amplitude of the area 1320 isin comparison to the peripheral area 1321 of the diaphragm 1302. Thecentral area 1320 includes a point of the diaphragm 1302 that providesthe largest vibration magnitude or amplitude when the diaphragm 1302receives the sound pressure.

As shown in FIG. 13 , the stepped contour surface 1308 of the cavityincludes three steps 1310, 1312, 1314. The lowest level step is 1310 isfacing at or around a central area 1320 of the diaphragm 1302. Thehighest level step 1314 is facing a peripheral area 1321 of thediaphragm 1302. In some embodiments, the highest level step 1314 isproximate to the anchors (the point of attachment) of the diaphragm1302. The middle level step 1312 is disposed between the lowest levelstep 1310 and the higher level step 1314.

The lowest level step 1310 includes one or more vent holes with a firstdensity (δ1). The middle level step 1312 includes one or more vent holeswith a second density (δ2). The highest level step 1314 includes one ormore vent holes with a third density (δ3). The first density is greaterthan the second density, and the second density is greater than thethird density, expressed as δ1>δ2>δ3. Because the lowest level step 1310is facing at or around a central area 1320 of the diaphragm 1302 wherethe vibration magnitude or amplitude is the largest, the highest density(δ1) of vent holes reduces the ambient noise. However, too many ventholes may reduce the surface area of the fixed plate 1304, andsubsequently reduce the capacitance value according to C=εA/d. Thus, thehigher level step 1314 with the lowest density δ3 of vent hole faces theperipheral areas 1321 of the diaphragm 1302 where the vibrationmagnitude or amplitude is minimum and does not significantly affect thereduction of ambient noise yet maintains the surface area of the fixplate 1304. The density distribution δ1>δ2>δ3 as show in FIG. 13maximizes the reduction of ambient noise and minimizes the reduction ofcapacitance value according to C=εA/d at the same time.

FIG. 14 shows a sectional view of a mesa diaphragm 1402 and a contour(e.g., stepped) cavity design. The mesa diaphragm 1402 is attached tothe first isolation layer 1404 at the anchors 1456. The first isolationlayer 1404 is attached to the first substrate 1406.

The outer surface 1492 of the mesa diaphragm 1402 faces the steppedcontour surface 1412 of the second substrate (e.g., fix plate) 1408. Thesecond isolation layer 1410 is attached to the second substrate 1410.

The first isolation layer 1404 and the second isolation layer 1410 arebonded together 1454 (the arrow shows the bonding orientation). In atleast one embodiment, the bonding involves forming covalent bondsbetween the two isolation layers 1404 and 1410.

The mesa diaphragm 1402 has a profile or cross-sectional shape similarto the shape of a trapezoid without a base. The mesa diaphragm 1402includes an inner surface 1490. In at least one embodiment, the innersurface 1490 faces the first cavity 1434. The mesa diaphragm 1402includes an outer surface 1492. The outer surface 1492 faces the steppedcontour surface 1412. In at least one embodiment, a top boarder of themesa diaphragm 1402 is entirely located within the second cavity 1432. Aportion of the second cavity 1432 is connected to the first cavity 1434.From a bottom side, the second cavity 1432 starts from a bottom surfaceof the second substrate 1408 and extends up to a recessed surface of thesecond substrate 1408. The first cavity 1432 is recessed in both thefirst substrate 1406 and 1408.

In a sectional view such as FIG. 14 , the mesa diaphragm 1402 has ashape similar to the shape of a trapezoid without a base. The mesadiaphragm 1402 includes a top boarder 1470 and two side boarders 1472.The bottom ends of the side boarders 1472 have anchors 1456. In at leastone embodiment, the anchors 1456 are attached to the first silicondioxide layer 1404. The side boarder 1472 joins the top boarder 1470 ata corner 1462. The corner 1462 has an angle α 1474 at the inner surface1490. In at least one embodiment, the angle α 1474 is larger than 70°,80°, 90°, 100°, or 110°. The top boarder 1470 have a top surface that isat the outer surface 1492 and a bottom surface that is at the innersurface 1490. The side boarder 1472 has an inner surface that is at theinner surface 1490 and an exterior surface that is at the outer surface1492.

As shown in FIG. 14 , the microphone 1400 includes a mesa diaphragm1402. The mesa diaphragm vibrates 1450 in a perpendicular direction whensound pressure reaches the mesa diaphragm 1402. The mesa diaphragm hasone or more anchors 1456 attached to the first silicon dioxide layer1404. The center portion 1421 of the mesa diaphragm 1402 can freelyvibrate with the sound pressure. The center portion 1421 of the mesadiaphragm 1402 vibrates in a direction that is perpendicular to a topboarder of the mesa diaphragm 1402. The diaphragm 1402 is structuredsuch that it does not touch a surface of the second substrate 1408during vibration 1450. During vibration 1450, the center portion 1421provides the largest vibration magnitude compared to the rest of theportion of the mesa diaphragm 1402.

The first silicon dioxide layer 1404 and the second silicon dioxidelayer 1410 are bonded 1454 together (the arrow of 1454 shows the bondingorientation). The top surface of the first silicon dioxide layer 1404 isa bonding surface. The bottom surface of the second silicon dioxidelayer 1410 is another bonding surface. The two bonding surfaces arecovalently bonded through fusion bonding process, e.g., baking thedevice at or about 950° C. to 1150° C. for a period of time. In anotherembodiment, the baking temperature is 1050° C. Under such temperature,the molecules in both silicon dioxide layers fuses across the bondingsurface and merges into a single structure. Therefore, the top surfaceof the first silicon dioxide layer 1404 and the bottom surface of thesecond dioxide layer 1410 are covalently bonded.

In at least one embodiment, the second silicon dioxide layer 1410 mayhave a vent hole (not shown in FIG. 14 . See vent hole 964 in FIG. 9Bfor example) going through the second silicon dioxide layer 1410. Thevent hole would fluidically connect the second cavity 1432 and anexterior of the microphone 1400. The vent hole may run in a directionparallel to a top and/or bottom surface of the first 1406 and/or second1408 semiconductor substrate.

The second semiconductor substrate 1408 includes one or more vent holes1462. The vent holes 1462 fluidically connect the first cavity 1432 andan exterior of the microphone 1400. The vent hole 1462 runs in adirection perpendicular to a top and/or bottom surface of the first 1406and/or second 1408 semiconductor substrate. The first semiconductorsubstrate 1406 includes the first cavity 1434. The second semiconductorsubstrate includes the second cavity 1408.

In FIG. 14 , the contour 1409 of the second semiconductor substrate 1408is a stepped contour surface 1412. A stepped contour surface 1412 may bemanufactured in a semiconductor processing. It is noted FIG. 14 does notpresent a limitation to any of the embodiments in this disclosure toinclude a smooth concave surface at a surface of the secondsemiconductor substrate 1408 facing the diaphragm.

The stepped contour surface 1412 has three levels of steps 1420, 1422,1424, wherein the bottom level 1420 (the lowest level) of the steppedcontour surface 1412 is centered around or facing a central area 1421 ofthe diaphragm 1402 that has the largest vibration magnitude. Here, thelargest vibration magnitude of the area 1421 is in comparison to therest of the area of the diaphragm 1402. The central area 1421 includes apoint of the diaphragm 1402 that provides the largest vibrationmagnitude when the diaphragm 1402 receives the sound pressure. In atleast one embodiment, the area 1421 of the diaphragm 1402 that has thelargest vibration magnitude is at or around a central area of thediaphragm 1402.

As shown in FIG. 14 , the stepped contour surface 1412 cavity includesthree steps 1420, 1422, 1424. The lowest level step is 1420 is facing ator around a central area 1421 of the diaphragm 1402. The highest levelstep 1424 is facing a peripheral area of the diaphragm 1402. In someembodiments, the highest level step 1424 faces the anchors 1456 of thediaphragm 1402. The middle level step 1422 is disposed between thelowest level step 1420 and the higher level step 1424.

The lowest level step 1420 includes one or more vent holes 1462 with afirst density (δ1). The middle level step 1422 includes one or more ventholes 1462 with a second density (δ2). The highest level step 1424includes one or more vent holes 1462 with a third density (δ3). Thefirst density is greater than the second, and the second is greater thanthe third, expressed as δ1>δ2>δ3.

FIGS. 15A-15O show a fabrication method 1500 to make a capacitiveacoustic transducer with mesa diaphragm and contoured fix plate suitablefor a microphone. The fabrication method 1500 can be implemented withany variety of conventional fabrication machines or processes (e.g.,depositing, etching, masking, planarizing), many or all of which may beautomated via a fabrication system. In at least one embodiment, themethod 1500 can be used to make a microphone 1400.

As illustrated in FIG. 15A, the second substrate 1502 is provided orprepared. The second substrate 1502 includes what will constitute anelectrically conductive plate of a capacitive sensor, which plate may befixed or non-moving with respect to other structures. The secondsubstrate 1502 comprises one or more layers of electrically conductive,electrically semiconductive, or electrically insulated material,although at least a portion that forms the plate is electricallyconductive and can maintain an electrical potential when a voltage isapplied across the plate and the mesa diaphragm via a voltage source.

As illustrated in FIG. 15A, a masking layer (lithography layer) 1504 isdeposited and patterned via one or more depositing and patteringprocesses to form a mask. The lithography layer 1504 has an opening foretching a recession into the second substrate 1502 to form a level ofthe stepped contour. The second substrate 1502 is similar to the secondsemiconductor substrate 1408 in FIG. 14 .

As illustrated in FIG. 15B, the second substrate 1502 is etched via oneor more etching processes, forming the first step 1506, which is thestep that faces peripheral area of the mesa diaphragm, similar to 1424in FIG. 14 . The first step 1506 is similar to the highest level step1424 in FIG. 14 . The recession formed in FIG. 15B is a portion of thesecond cavity 1521.

As illustrated in FIG. 15C, another masking layer (not shown) isdeposited and patterned via one or more depositing and patterningprocesses to form a mask for etching the second step 1508 of the contoursurface. The masked second substrate 1502 is further etched via one ormore etching processes to form second step 1508. The second step 1508 issimilar to the middle level step 1422 in FIG. 14 . The recession formedin FIG. 15C is another portion of the second cavity 1521.

As illustrated in FIG. 15D, yet a further masking layer (not shown) isdeposited and patterned via one or more depositing and patterningprocesses to form a further mask for etching the third step 1510 of thecontour surface. The masked second substrate 1520 is further etched toform a third step 1510. The third step 1510 is the step that faces thecentral portion of the mesa diaphragm, similar to the lowest level step1420 in FIG. 14 . The recession created in FIG. 15D is a portion of thesecond cavity 1521. Additional masking and patterning operations may beperformed depending on the total number of steps that are required toachieve a desired level of smoothness in the contour surface.

The method 1500 may include depositing or forming or growing anelectrical insulation layer, for example forming or growing a layer ofsilicon dioxide 1512 via one or more oxidation processes. The silicondioxide layer 1512 is similar to the second silicon dioxide layer 1410in FIG. 14 . The second cavity 1521 created in FIG. 15A-15D is includedin the second substrate 1502.

As illustrated in FIG. 15E, a masking layer 1514 is deposited andpatterned on the first substrate 1516 via one or more depositing andpatterning processes to form a mask for etching the first cavity 1523.The first substrate 1516 is similar to or constitutes the firstsemiconductor substrate 1406 (FIG. 14 ).

As illustrated in FIG. 15F, the masked first substrate 1516 is etched toform a cavity therein. As also illustrated in FIG. 15F, an electricalinsulating layer is deposited, formed or grown, for example bydepositing, forming or growing a layer of silicon dioxide 1518 via oneor more oxidation processes. The silicon dioxide layer 1518 is similarto the first silicon dioxide layer 1404 in FIG. 14 . The first cavity1523 is created by an etching operation of a semiconductor process tocreate a recess in the first substrate 1516. The first cavity 1523created in FIG. 15F will be later extended to a bottom surface of thefirst substrate 1516. See FIG. 15N.

As illustrated in FIG. 15G, a layer of polysilicon layer 1520 isdeposited on top of the silicon dioxide layer 1518 within the firstcavity 1523, via one or more depositing processes (e.g., chemical-vapordeposition).

As illustrated in in FIG. 15F, the layer of polysilicon layer 1520 isetched via one or more etching processes. The polysilicon layer 1520 issimilar to the mesa diaphragm 1402 in FIG. 14 . This polysilicon layer1520 will later become the mesa diaphragm in FIG. 15N.

As illustrated in FIG. 15H, the second silicon dioxide layer 1512 andthe first silicon dioxide layer 1518 are attached or secured together,for example via thermal bonding, also known as fusion bonding. Thebonding surface 1522 is covalently bonded. The first cavity 1523 isincluded in the first substrate 1516. The second cavity is included inthe second substrate 1502, and is aligned or in registration with thefirst cavity 1523 when the two substrates are joined together. Thedashline shown in FIG. 15H between the first cavity 1523 and the secondcavity 1521 is an imaginary boundary line between the first cavity 1523and the second cavity 1521.

As illustrated in FIG. 15I, the second substrate 1502 ground or polishedto a desired thickness via one or more material removal processes (e.g.,planarization, chemical-mechanical planarization).

As illustrated in FIG. 15J, a masking layer 1526 is deposited andpatterned, via one or more depositing and patterning processes, tocreate a mask for fabricating one or more slots for electrical contacts1528 and 1529. Electrical contact 1528 is the electrical contact for thesecond substrate 1502. The electrical contact 1529 is the electricalcontact for the polysilicon layer 1520. The electrical contacts 1528 and1529 may be employed to apply an electrical potential to the diaphragm(e.g., polysilicon layer 1520) and the fixed plate (e.g., the secondsubstrate 1502) opposed across a space from the diaphragm to form acapacitor, and to couple a capacitance sensor circuit thereacross tosense a capacitance or change in capacitance and the distance betweenthe diaphragm and fixed plate varies in response to oscillation of thediaphragm. A shallow channel may be etched in the first substrate 1502to form the electrical contact 1528. A deep channel may be etchedthrough the first substrate 1502 and into the second substrate 1516 toform the electrical contact 1529.

As illustrated in FIG. 15K, low temperature oxide (LTO) electricallyinsulative layer (e.g., silicon dioxide layer 1530) is deposited, formedor grown. This silicon dioxide layer 1530 is deposited, formed or grown,from an exterior surface of the second substrate 1502 and into the wellof electrical contacts 1528 and 1529.

As illustrated in FIG. 15L, a masking layer 1531 is deposited andpatterned via one or more depositing and patterning processes to form amask for fabricating the vent holes 1536. The masked silicon dioxidelayer 1530 is etched via one or more etching processes to fabricate thevent holes 1536. The vent holes 1536 are similar to, and may constitute,the vent holes 1462 in FIG. 14 .

As illustrated in FIG. 15L, the silicon dioxide layer 1530 within abottom surface of the deep well of the electrical contact 1529 is etchedto expose a portion of the polysilicon layer 1520. The polysilicon layer1520 is electrically conductive or semiconductive material. As alsoillustrated, a polysilicon layer 1533 is deposited into the well ofelectrical contact 1529 via one or more depositing processes. Thepolysilicon layer 1533 is in electrical contact with the polysiliconlayer 1520. The dashline shown in FIG. 15L between the first cavity 1523and the second cavity 1521 is an imaginary boundary line between thefirst cavity 1523 and the second cavity 1521.

As illustrated in FIG. 15M, metal layers 1532 are deposited via one ormore depositing processes to form the electrical contacts 1529 and 1528.The metal layer 1532 of electrical contact 1528 is in electrical contactwith the second substrate 1502. The metal layer 1532 of electricalcontact 1529 is in electrical contact with the polysilicon layer 1520.The dashline shown in FIG. 15M between the first cavity 1523 and thesecond cavity 1521 is an imaginary boundary line between the firstcavity 1523 and the second cavity 1521.

As illustrated in FIG. 15N, a masking layer (not shown) is deposited andpatterned at a bottom surface of the first substrate 1516 to form a maskfor fabricating another portion of the first cavity 1523. The method1500 includes etching through the masked first substrate 1516 from thebottom surface of the first substrate 1516 up to the silicon dioxidelayer 1518.

Further, the method 1500 includes etching through the silicon dioxidelayer 1518 up to the polysilicon layer 1520. When the silicon dioxidelayer 1518 within the first cavity 1523 is etched away, the polysiliconlayer 1520 is flipped upward to form a mesa diaphragm. This flipping canbe a mechanical force generated by the molecular structure of thepolysilicon layer 1520 under a specific environmental condition, e.g.,temperature. This process is also previously referred to in thisdisclosure as releasing the diaphragm (e.g., polysilicon layer 1520).The polysilicon layer 1520 in mesa formation includes all the featuresof the mesa diaphragm 1402 and mesa diaphragms 902.

The dashline shown in FIG. 15N between the first cavity 1523 and thesecond cavity 1521 is an imaginary boundary line between the firstcavity 1523 and the second cavity 1521. As illustrated in FIG. 15N, thefirst cavity 1523 is extended to extend through an exterior of the firstsubstrate 1516.

As illustrated in FIG. 15O, a masking layer (not shown) is deposited andpatterned via one or more depositing and patterning processes to form amask for fabricating the vent holes 1536 with the desired densitydistributions (δ). The method 1500 includes etching through the maskedsecond substrate 1502 to fabricate the vent holes 1536. The method 1500includes etching through the silicon dioxide layer 1512. The method 1500includes forming the vent holes 1536 with the desired densitydistribution. In at least one embodiment, the vent holes 1536 aredistributed as the embodiment in FIG. 14 , having a higher vent holedensity (δ) in a central area wherein the vent hole density (δ)laterally decreases toward the peripheral areas. The dashline shown inFIG. 15O between the first cavity 1523 and the second cavity 1521 is animaginary boundary line.

The various embodiments disclosed herein include specific designs forcapacitive acoustic transducer or sensor that includes an electricallyconductive diaphragm and a second substrate opposed to the diaphragmacross a cavity, at least a portion of the second substrate beingelectrically conductive. One or more dielectric materials are includedin-between the diaphragm and the second substrate. In some embodiments,the diaphragm is in mesa form and the second substrate has a cavity witha non-planar contoured or structured bottom surface that faces thediaphragm that improves or even maximizes a signal to noise ratio (SNR)of a the transducer or microphone. The mesa diaphragm design providesadvantageous properties such that the capacitance value (C) isindependent from the thickness of isolation layers. This meansincreasing the isolation layer will not decrease the capacitance value(C). Therefore, thickness of isolation layers can be increased to reducethe leaking current (white noise), without compromising the capacitancevalue. The contoured or structured bottom surface of the cavity of thesecond substrate provides advantageous properties that maximize thesensitivity of the diaphragm vibrations, which translates to high SNR ofthe transducer or microphone.

While aspects of the present disclosure have been described inconjunction with the specific embodiments thereof that are proposed asexamples, alternatives, modifications, and variations to the examplesmay be made. Accordingly, embodiments as set forth herein are intendedto be illustrative and not limiting. The various implementations and/orembodiments described above can be combined to provide furtherimplementations and/or embodiments. There are changes that may be madewithout departing from the scope of the claims set forth below. Theseand other changes can be made to the implementations and/or embodimentsin light of the above-detailed description. In general, in the followingclaims, the terms used should not be construed to limit the claims tothe specific implementations and/or embodiments disclosed in thespecification and the claims, but should be construed to include allpossible implementations and/or embodiments along with the full scope ofequivalents to which such claims are entitled. Accordingly, the claimsare not limited by the disclosure.

What is claimed:
 1. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive, at least a portion of the mesa diaphragm moveable along an oscillation axis, and an anchor of the mesa diaphragm that secures the diaphragm to another of the two or more layers of the first substrate positioned within the first cavity; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where a flat top border of the mesa diaphragm is positioned within the second cavity, at least a portion of the second substrate is electrically conductive, and the cavity bottom surface is non-planar.
 2. The capacitive micro-electromechanical transducer of claim 1 wherein a depth of the second cavity as measured perpendicularly from the second opening to the cavity bottom surface increases as the cavity is laterally traversed from a perimeter thereof to a center thereof.
 3. The capacitive micro-electromechanical transducer of claim 1 wherein the cavity bottom surface comprises a plurality of stepped regions, a depth of the second cavity as measured perpendicularly from the second opening to the cavity bottom surface increases as the cavity is laterally traversed from a perimeter thereof to a center thereof.
 4. The capacitive micro-electromechanical transducer of claim 1 wherein, at least when static, the mesa diaphragm extends longitudinally inwards along the oscillation axis, toward the cavity bottom surface.
 5. The capacitive micro-electromechanical transducer of claim 4 wherein a maximum perpendicular distance between the mesa diaphragm and the cavity bottom surface renders a value of capacitance of the capacitive micro-electromechanical transducer independent of a thickness of an electrical isolation.
 6. The capacitive micro-electromechanical transducer of claim 1 wherein the second substrate includes a plurality of holes that extend from the cavity bottom surface through the exterior surface of the second substrate.
 7. The capacitive micro-electromechanical transducer of claim 6 wherein the plurality of holes that extend from the cavity bottom surface through the exterior surface of the second substrate are non-uniform in at least one of size or distribution, where a relative density of the holes or a relative size of the holes increasing as the cavity bottom surface is laterally or radially traversed from a perimeter thereof to a center thereof.
 8. The capacitive micro-electromechanical transducer of claim 6 wherein the holes of the plurality of holes are uniform is size in a lateral dimension, and a density of the holes is higher at a center than at a perimeter of the cavity bottom surface.
 9. The capacitive micro-electromechanical transducer of claim 1 wherein at least one of the first or the second substrates includes a vent that extends laterally from the second cavity to an exterior of at least one of the first or the second substrates to vent a chamber formed by the cavity and the mesa diaphragm.
 10. The capacitive micro-electromechanical transducer of claim 1 wherein the first substrate includes an electrically conductive line electrically coupled to the mesa diaphragm.
 11. The capacitive micro-electromechanical transducer of claim 1 wherein the first substrate includes an electrically conductive line electrically coupled to the electrically conductive portion of the second substrate.
 12. The capacitive micro-electromechanical transducer of claim 1 wherein the first substrate includes a wafer and at least one oxide layer carried by the wafer at least proximate the inner surface of the first substrate.
 13. The capacitive micro-electromechanical transducer of claim 1 wherein the first substrate includes a wafer and a first oxide layer carried by the wafer, and the second substrate includes an electrically conductive or semiconductive layer and a second oxide layer carried by the electrically conductive or semiconductive layer.
 14. The capacitive micro-electromechanical transducer of claim 13 further comprising: a fusion bond that secures the second substrate to the first substrate via the first and the second oxide layers.
 15. A microphone, comprising: a capacitive micro-electromechanical transducer; and a packaging that houses the capacitive micro-electromechanical transducer, the housing having at least two contacts on an exterior thereof, wherein the capacitive micro-electromechanical transducer comprises: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that spans the first cavity, a first portion of the mesa diaphragm moveable along an oscillation axis with respect to at least one layer of the first substrate, and a second portion of the mesa diaphragm secured to another of the at least one layers at a location that is within the first cavity; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with a second opening at least proximate the interior surface of the second substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the second opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis at least partially into the second cavity, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar.
 16. The microphone of claim 15, further comprising: a capacitance sensor circuit electrically coupled to the mesa diaphragm and electrically coupled to at least the portion of the second substrate that is electrically conductive to sense a change in capacitance as the mesa diaphragm vibrates.
 17. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein, at least when static, the mesa diaphragm extends longitudinally inwards along the oscillation axis, toward the cavity bottom surface, and wherein a maximum perpendicular distance between the mesa diaphragm and the cavity bottom surface renders a value of capacitance of the capacitive micro-electromechanical transducer independent of a thickness of an electrical isolation.
 18. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein the second substrate includes a plurality of holes that extend from the cavity bottom surface through the exterior surface of the second substrate, and the plurality of holes that extend from the cavity bottom surface through the exterior surface of the second substrate are non-uniform in at least one of size or distribution, where a relative density of the holes or a relative size of the holes increasing as the cavity bottom surface is laterally or radially traversed from a perimeter thereof to a center thereof.
 19. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein the second substrate includes a plurality of holes that extend from the cavity bottom surface through the exterior surface of the second substrate, and the holes of the plurality of holes are uniform is size in a lateral dimension, and a density of the holes is higher at a center than at a perimeter of the cavity bottom surface.
 20. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein at least one of the first or the second substrates includes a vent that extends laterally from the second cavity to an exterior of at least one of the first or the second substrates to vent a chamber formed by the cavity and the mesa diaphragm.
 21. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein the first substrate includes a wafer and at least one oxide layer carried by the wafer at least proximate the inner surface of the first substrate.
 22. A capacitive micro-electromechanical transducer, comprising: a first substrate comprised of two or more layers and having an exterior surface, an interior surface, and a first cavity with a first opening at the exterior surface of the first substrate and a second opening at least proximate the interior surface of the first substrate, at least one of the layers of the first substrate comprising a mesa diaphragm that is electrically conductive and that extends across the second opening of the first cavity, at least a portion of the mesa diaphragm moveable along an oscillation axis; and a second substrate comprised of at least one layer and having an exterior surface, an interior surface, and a second cavity with an opening at least proximate the interior surface of the first substrate and a cavity bottom surface, the interior surface of the second substrate secured to the interior surface of the first substrate with the mesa diaphragm in registration with the opening with at least a portion of the mesa diaphragm positioned to oscillate along the oscillation axis between the first and the second cavities, where at least a portion of the second substrate is electrically conductive and the cavity bottom surface is non-planar, wherein the first substrate includes a wafer and a first oxide layer carried by the wafer, and the second substrate includes an electrically conductive or semiconductive layer and a second oxide layer carried by the electrically conductive or semiconductive layer.
 23. The capacitive micro-electromechanical transducer of claim 22, further comprising: a fusion bond that secures the second substrate to the first substrate via the first and the second oxide layers. 